If that's what it was, he's already dead. Just moving around and not aware he's dead yet.
If you want to be nice, call your roommate and see what he/she says about it. A vet could probably make him puke it up.
Cats cannot "spit" something out, they toss things around in their mouth and it usually falls out. A dry pill with a wet tongue basically means it's getting swallowed because it won't fall. They rely on fantastic upchuck ability to get rid of items ingested that way.
Also, recreational prescription drugs are bad, mkay? The type of place with roommates and ambien on the floor seems like one step away from a felony stop and loss of firearms. The only people I know that have pills go missing randomly (in couch cushions, floor, etc.) are taking drugs relationally and mixing them with booze. Maybe clean up your act? and your apartment for that matter.